One of the services provided in the oil field service industry pertains to the stimulation of an oil or gas bearing formation after a well hole has been drilled. Upper and lower plugs or packers are inserted in the well above and below perforations in the well casing, which provide access to the subsurface formation. A fracturing fluid is pumped into the well between the two packers and is forced, under high pressure, through the casing perforations into the formation. It is intended that the fluid will cause the formation to fracture outwardly from the perforations, providing channels for the oil or gas to flow into the well.
Once fractures have opened, a proppant is added to the fracturing fluid to be injected into the fractures. The proppant, which can be sand, bauxite, or other like material, props open the fractures to prevent them from closing when pumping of the fracturing fluid ceases and the pressure on the fractures is reduced. It can be appreciated that too little proppant may allow the fractures to close, thereby reducing the flow of oil or gas into the well. It can also be appreciated that too much proppant can clog the fractures, as well as the bore hole, fluid lines, pumps and valves, thereby also adversely affecting production and increasing the required maintenance.
To produce proppant-laden fracturing fluid (slurry), a "clean" base fluid is continuously pumped into a blending tank and a proppant is continuously added. Proppant-laden slurry is discharged from the blender and is pumped under high pressure through a pipe or flow line into the well. The amount of proppant added must be carefully monitored and controlled to ensure that production of the well is optimized.
One common device used to monitor the amount of proppant which has been added to the base fluid is a nuclear densitometer (or "densometer") which measures the density of the slurry being discharged from the blender. A radiation source, such as Cesium 137, is positioned against one side of the discharge flow line and a radiation detector is positioned against the opposite side. The radiation is directed through the first side of the flow line, through the discharged fluid, and through the opposite side of the flow line to the detector. The amount of radiation which actually reaches the detector is proportional to the density of the fluid: if the relative amounts of all other components in the slurry remain constant, the greater the density of the slurry (i.e., the more proppant in the slurry), the more radiation will be absorbed in the slurry and the less will be detected. The output signal from the detector can be processed and the density, in units such as pounds of sand added per gallon fluid (PSA), specific gravity units (SGU), or pounds per gallon (PPG) can be displayed for the operators.
A nuclear densitometer has many attendant disadvantages, one of which is its reliance upon a radiation source. It is necessary for the operator to have federal and, possibly, state licenses and be subject to extensive regulations. Handling and transportation of the device, while not dangerous, is subject to prescribed procedures. Additionally, as can be appreciated, the radiation source irradiates a portion of the fluid flow line to which the densitometer is attached, causing it to become radioactive. Consequently, that portion of the flow line must be handled and transported with as much care as the nuclear densitometer itself.
Other operational disadvantages include the need for separate units for different-size flow lines. Less radiation will be detected through an 8-inch flow line than through a 6-inch flow line, even if the density of the fluid is the same. Consequently, a different densitometer unit is usually used. Additionally, component age, temperature variations, and the decay of the radioactive source tend to cause the device to "drift." Consequently, various compensation techniques must be employed to prevent such drifting from being interpreted as increased or decreased fluid density. Finally, circuitry associated with a nuclear densitometer may be noisy and slow, resulting in inaccuracies and delays in the monitoring and control of the amount of proppant being added.
Consequently, a need has arisen for an apparatus and method for determining the amount of material added to a flowing fluid which substantially reduce or avoid the foregoing disadvantages.